1. Field of the Invention
The present invention relates to bi-directional communication systems that perform bi-directional communication of OFDM signals between opposing apparatuses using a predetermined transmission frequency band.
2. Description of the Related Art
In recent years, orthogonal frequency division multiplexing (OFDM) transmission formats have been applied in such areas as terrestrial digital television broadcasting. OFDM transmission formats involve transmitting transmission data at low speeds in parallel using multiple orthogonal sub-carriers, and therefore have characteristics such as multipath resistance and high efficiency in frequency usage.
Japanese Patent Laid-Open No. 11-055211 (hereinafter, patent document 1) describes a method in which bi-directional communication is performed between opposing apparatuses using an FDD (frequency division duplex) method on signals that have been modulated by the above-mentioned OFDM. Patent document 1 discloses a bi-directional communication method in which OFDM signals are transmitted in a reverse direction in synchronization with received OFDM signals. According to the transmission method described in patent document 1, the OFDM signals transmitted by each apparatus are orthogonal and the signals do not interfere with each other during reception demodulation. That is, by assigning frequency bands adjacent to each other, it is possible to expand the bi-directional transmission bandwidth.
However, no consideration is given in patent document 1 to the signal transmission times. For this reason, the following problems occur.
FIG. 8 is a block diagram of a bi-directional communication system according to patent document 1 in which a communication apparatus 801 and a communication apparatus 802 are provided with a transmission/reception unit 803 and a transmission/reception unit 804, and carry out bi-directional communication. It should be noted that the bandwidth of the communication frequency (sub-carrier frequency) used by the transmission/reception units 803 and 804 are assigned adjacent to each other as shown in FIG. 9. Furthermore, hybrid circuits or the like are not provided in the transmission/reception units 803 and 804, and in addition to being received by the transmission/reception unit 804 (803), the signals transmitted from a transmitter of the transmission/reception unit 803 (804) are fed back and received by a receiver of the transmission/reception unit 803 (804). Numeral 805 indicates a transmission channel. The transmission channel 805 is indicated using a solid line, however it is of no concern whether it is wired or wireless as a physical transmission channel. Numeral 806 indicates an OFDM signal that the transmission/reception unit 803 transmits to the transmission/reception unit 804 (hereinafter referred to as a downlink OFDM signal 806), and numeral 807 indicates an OFDM signal that the transmission/reception unit 804 transmits to the transmission/reception unit 803 (hereinafter referred to as an uplink OFDM signal 807).
In FIG. 10, numerals 1001 and 1002 indicate time assignments of the downlink OFDM signal 806 and the uplink OFDM signal 807 received by the transmission/reception units 803 and 804. It should be noted that although consideration is given in FIG. 10 to a transmission time Td of signals between the transmission/reception units 803 and 804, the transmission times of signals relating to the feedback are considered short enough to be ignorable. The transmission/reception units 803 and 804 transmit and receive OFDM symbols, which are constituted by an effective symbol and a guard interval (GI) in which a predetermined period of an end portion of the effective symbol is copied. The downlink OFDM signal 806 that is transmitted from the transmission/reception unit 803 is received by the transmission/reception unit 804 as the downlink OFDM signal 1005 after the transmission time Td from transmission, and is also fed back and received by the transmission/reception unit 803 (downlink OFDM signal 1003).
According to patent document 1, the transmission/reception unit 804 transmits the uplink OFDM signal 807 in synchronization with the downlink OFDM signal 1005. At this time, the uplink OFDM signal 807 that is transmitted from the transmission/reception unit 804 is received by the transmission/reception unit 803 as an uplink OFDM signal 1004 after the transmission time Td from transmission, and is also fed back and received by the transmission/reception unit 804 (uplink OFDM signal 1006). Here, in a case where the effective symbol period of the downlink OFDM signal 1005 of the transmission/reception unit 804 is used as a demodulation processing period (1007), the downlink OFDM signal 1005 and the uplink OFDM signal 1006 are orthogonal, and therefore the downlink OFDM signal 1005 can be demodulated without interference.
On the other hand, with the transmission/reception unit 803, the symbol timings of the downlink OFDM signal 1003 and the uplink OFDM signal 1004 have a time difference of 2Td, that is a round-trip transmission time, as indicated by numeral 1001. For this reason, in a case where the effective symbol period of the uplink OFDM signal 1004 of the transmission/reception unit 803 is used as a demodulation processing period (1008), a symbol boundary of the downlink OFDM signal 1003 is included, which impairs orthogonality. That is, problems are caused in that interference is produced and reception capabilities are reduced.
It should be noted that it is possible to maintain the orthogonality of both OFDM signals by setting the demodulation processing period for the transmission/reception unit 803 to precede (1009) the effective symbol period of the uplink OFDM signal 1004. However, patent document 1 has no description relating to such control of the periods of demodulation processing. Furthermore, when adjusting the demodulation processing periods in this manner, the period that functions as the GI in the received signals of the transmission/reception unit 803 becomes shorter compared to that of the received signals of the transmission/reception unit 804, which greatly reduces multipath resistance. Accordingly, there is a need to set the guard interval length (GI length) longer in order to avoid multipath interference in the transmission/reception unit 803, but in this case a problem is caused in that the bi-directional transmission bandwidth is reduced.